This invention relates generally to a system and method for color normalization of digital camera images of documents. More particularly, the system and method are directed to enhancing the colors of digital camera images to correct for uneven lighting and contrast in such images.
While the invention is particularly directed to the art of image processing and color normalization, and will thus be described with specific reference thereto, it will be appreciated that the system and method disclosed herein may have usefulness in other fields and applications.
Document images, such as those of a whiteboard, blackboard, flip chart, drawings, or other images captured with a digital camera may suffer from uneven lighting and contrast. Image processing is required to correct for these effects and return an image to a form more comparable to that provided by a true document scanner. Existing techniques for automatic white balancing and contrast adjustment of these images, such as Autobalance in Microsoft Photo Editor, perform inadequately to this task because they apply a color tone adjustment uniformly across the image. Because the original illumination is non-uniform, color adjustment parameters are required that are non-uniform across the image. By way of illustration, FIG. 1 is a digital camera image of a hand-drawn artwork. Due to low and uneven illumination and the camera's estimate of white-balance, the digital camera image appears dark, with an uneven brown background, when compared to the original shown in FIG. 2. After processing the image of FIG. 1 with Autobalance, color is adjusted, as shown in FIG. 3, but the contrast adjustment is not sufficient when compared to FIG. 2. The background retains a yellow cast and the colors of the animals and their blankets is not in agreement with the original image shown in FIG. 2.
Techniques for color correcting images of whiteboards which are illuminated unevenly perform well for the thin linework made by marker pens, but fail when the image content consists of solid color regions as well as lines. An example of this is the commercial product, Whiteboard Photo, which enhances linework, but fails to adequately enhance solid color regions. For example, FIG. 4 is the output for the original artwork shown in FIG. 2 after processing with Whiteboard Photo, while FIG. 5 is the result of color normalization using the color normalization technique disclosed in U.S. patent application Ser. No. 09/158,443, “System and Method for Color Normalization of Board Images,” and used in the ZombieBoard whiteboard scanner application described in the technical report, “Saund, E., ‘Bringing the Marks on a Whiteboard to Electronic Life’, at http:/Hwww.parc.com/saund/papers.html”.
In FIG. 4, the background is now white, but the solid portions of the animals' bodies and their blankets is uneven and not an accurate match with the original of FIG. 2. In FIG. 5, the version of the digital camera input after processing with the ZombieBoard technique, the background is also now white, but again the solid portions of the animals and their blankets is uneven and inaccurate when compared with FIG. 2. These systems rely on high-pass filtering techniques, or their equivalent, to distinguish foreground from the ostensibly white background, which is assumed to be a uniform white color illuminated by a lighting source that varies gradually over the surface. As a result, the interiors of solid regions appear to be washed out after processing. However, neither of these solutions extends the technology of color normalization to images that contain solid color regions as well as thin linework.
The present invention provides a new and improved image processing method for color images which overcomes the problems associated with uneven illumination and a camera's estimate of white-balance, extending the technology of color normalization to images that contain solid color regions. An advantage of this invention is that it classifies all image pixels as being either foreground or background. This classification can be useful to applications whether or not color enhancement of foreground material is performed, for example, to cause background pixels to be rendered as transparent for purposes of combining and rearranging foreground image material.